1. Field of the Invention
The present invention relates to a radio transmission access method suitable for, for example, such a case where various kinds of information are transmitted by using radio signals and a local area network (LAN) is formed among a plurality of devices, and a radio transmission device for conducting transmission by using this access method.
2. Description of the Related Art
Conventionally, in the case where a local area network is formed among a plurality of devices, such as various kinds of video devices, personal computer devices, and their peripheral devices, in a comparatively narrow range such as home or an office so as to be capable of transmitting data handled by those devices, data transmission is sometimes performed with radio transmission by connecting a radio signal transmission and receiving device (radio transmission device) to each device instead of directly connecting devices with some signal lines.
By forming a local area network as a system for performing radio transmission, it is not necessary to connect devices directly by using signal lines and the system configuration can be simplified.
In the case where a plurality of radio transmission devices are prepared and a local area network is formed, there is a risk of occurrence of transmission errors if signals are simultaneously transmitted from a plurality of radio transmission devices by using the same transmission band. Therefore, it is necessary to conduct access control on communication of transmission devices in the network by using some method.
Examples of the access control method heretofore known will now be described. For example, for a small-scale radio network, there is such a method that communication among transmission devices (peripheral terminal stations) in the network is unitarily managed by a transmission device (central control station) located in a center portion of star connection. To be concrete, there is considered such processing that transmission of data from another transmission device peripheral terminal station) is controlled by polling control from the transmission device (central control station) located in the center portion.
As a different access method, there is such a method that in, for example, a small-scale radio network, each communication station establishes a connection link between it and a communication station located around it and each communication station performs asynchronous communication as occasion demands. To be concrete, there is considered access control processing in which a certain communication station ascertains beforehand that transmission is not being conducted by another communication station and starts information transmission so as not to collide with another communication station in transmission state.
In access control processing in which information transmission is started so as not to cause a collision in transmission state with another communication station, it is necessary to conduct transmission and reception of information concerning communication stations existing in peripheral regions and information concerning position relations of those communication stations prior to information transmission and thereby establish in advance such a state that a network has been formed.
For example, in the case where polling from a transmission device (central control station) located in the central portion is conducted, a central control station needs to always transmit a polling signal onto the network. There is a fear that transmission of the polling signal may obstruct operation of other network systems.
In the case where such a method that a central control station unitarily manages communication, a radio wave is transmitted only when radio transmission has become necessary, and consequently the possibility of coexistence with other communication systems is increased. If a plurality of communication stations conduct information transmission at arbitrary timing, however, there is a risk of occurrence of a collision. Therefore, it is necessary to ascertain the network state when an information transmission request has occurred and secure a radio transmission channel in advance prior to transmission. This results in a problem that it takes a long time until actual information transmission.
On the other hand, in the case of such a method that communication stations conduct asynchronous communication as occasion demands, for control informations transmitted from each communication device not to occur a collision each other, the central control station needs to always send polling signals to peripheral terminal stations. The peripheral terminal stations also need to reply to the polling signals as occasion demands. Even in the case where there is no information to be transmitted, therefore, radio transmission channels are occupied.
In the peripheral terminal stations, it is also possible to adopt such a power saving system configuration that a reply is not given to a polling signal for setting that there is not a demand for transmission. In the control station, however, polling signals must be always transmitted, and consequently the power dissipation becomes enormous. In addition, it cannot be avoided on the network to interfere with other communication systems.
In view of these points, an object of the present invention is to make it possible to transmit necessary information by using a minimum of radio transmission channels in such a radio transmission network.
In accordance with a first aspect of the present invention, a network system using radio waves usually operates in a dormant state so as to be capable of coexisting with other communication systems. If the network system has detected a possibility of information transmission on a radio transmission channel based on behavior of a connected device or on a transmission channel for the device, then the network system constructs a radio network in advance prior to radio transmission and comes in an operational state.
According to the first aspect of the present invention, the radio network comes in a dormant state when information transmission in the radio network is not required only when there is a possibility of information transmission, the radio network comes in an operational state. By minimum operation of the radio network, necessary information transmission is conducted.
In accordance with a second aspect of the present invention, polling operation from a control station to each of peripheral terminal stations on a network is not executed and information transmission is not conducted, in a steady state of asynchronous radio transmission channels. If a transmission demand has occurred in any peripheral terminal station, then the peripheral terminal station transmits a polling request signal to a control station and causes polling operation from the central control station to be started.
According to the second aspect of the present invention, polling operation is conducted only when there is a possibility that each of the stations included in the radio network transmits information. With minimum polling operation, necessary information transmission is conducted.